In recent years, the demand for fuel cells in which chemical energy of a fuel is directly converted into electrical energy has been growing. In general, a fuel cell has a structure comprising a stack of many unit cells each comprising electrode plates, an electrolyte film sandwiched therebetween, and a separator disposed on an outer side of these.
FIG. 1 is a diagrammatic illustration showing the appearance of a general separator for fuel cells. This separator has a constitution in which a flat plate member 6 has first partition walls 7a protruding from one side thereof at a predetermined interval to form channels 8a through which a reactant gas (hydrogen or oxygen) is to be passed, and further has second partition walls 7b protruding from the other side thereof at a predetermined interval to form channels 8b through which cooling water is to be passed. Hereinafter, the side on which the channels 8a for passing a reactant gas therethrough have been formed is referred to as “gas passage-side”, while the side on which the channels 8b for passing cooling water therethrough have been formed is referred to as “cooling water passage-side” (see FIG. 2).
In fabricating a fuel cell, many such fuel-cell separators 5 are stacked in the direction of projection of the partition walls 7a and 7b (upward/downward direction in the figure). The airtightness of the channels 8a and 8b is hence important, and strength and dimensional accuracy are required of the partition walls 7a and 7b. In the field where a high voltage is required as in, e.g., motor vehicles, several hundred unit cells are stacked to fabricate a cell. Since this stack is generally used under the conditions of about 80° C., the separators are required to have dimensional stability to heat (low thermal expansivity). In case where the separators have high thermal expansivity, the stack as a whole expands thermally and the clamping load hence increases. As a result, the fuel-cell separators themselves and the electrolyte films break. There are also cases where due to the insufficient strength, insufficient plate-thickness dimensional accuracy, and warpage of the separators, etc., stack assembly encounters difficulties in assembly operation, breakage, and other troubles.
In particular, the warpage of fuel-cell separators not only makes stack assembly difficult but also results in insufficient contact between unit cells after assembly. As a result, contact electrical resistance becomes uneven, leading to a decrease in power generation performance. There are also cases where an offset load breaks the fuel-cell separators themselves.
On the other hand, fuel-cell separators 5 are generally obtained by molding into a predetermined shape a conductive resin composition comprising a resin and a conductive filler dispersed therein, since this method is advantageous in productivity. Examples of such methods which have been proposed include: a method comprising dry-blending a phenolic resin with an expanded graphite, filling the blend into a mold, and compression-molding it (see reference 1); a method in which a powdery mixture obtained by dry-blending a phenolic resin with an expanded graphite is compacted to produce a preform, and this preform is cured and compression-molded (see reference 2); and a method comprising kneading a phenolic resin together with a carbon powder by means of a pressure kneader or the like, and compression-molding the mixture to obtain a molding (see reference 3).
However, those methods in which a conductive resin composition is molded have the following disadvantages. As the sectional view of FIG. 2 shows schematically, there is a difference in the orientation of the conductive filler, e.g., expanded graphite, between the thickness direction (upward/downward direction on the page) and the direction perpendicular to the thickness direction (left/right direction on the page) due to a difference in shape between the gas passage-side and the cooling water passage-side. Hereinafter, the direction perpendicular to the thickness direction is simply referred to as the horizontal direction. As a result, differences in thermal expansion arise, and local differences in elongation hence occur, resulting in warpage or undulation of the separator. Techniques for warpage reduction which have been proposed include: a method in which a powdery mixture of a thermosetting resin and a carbon powder is press-molded with heating, and the resultant molding is released from the mold and then heated while being sandwiched between smooth-surface plates, at a temperature which is 50-100% of the curing temperature (see reference 4); and a method in which a high-strength high-rigidity material is used as a frame material (see reference 5).
[Reference 1] JP 2000-48830 A
[Reference 2] JP 2000-77081 A
[Reference 3] JP 2000-243409 A
[Reference 4] JP 2000-243409 A
[Reference 5] JP 2002-358973 A
However, those related techniques for warpage reduction are ineffective in eliminating the differences in thermal expansion coefficient attributable to the difference in conductive-filler orientation between the gas passage-side and the cooling water passage-side, and the degree of warpage reduction attainable therewith is limited. Although use of a separator having a high elastic modulus can reduce warpage or undulation, the high elastic modulus may raise problems such as breakage, chipping, etc. in stack assembly.
An object of the invention is to provide a separator for fuel cells which is reduced in warpage. Another object of the invention is to provide a process by which a fuel-cell separator reduced in warpage is produced by a relatively easy economical method.